Vehicles can be selectively powered using electrical energy in order to improve overall fuel economy, as well as to reduce levels of various vehicle emissions. In order to store sufficient amounts of electrical energy for propelling the vehicle over an adequate range, an electrical storage device or ESD such as a battery, battery pack, or other electrochemical energy storage device can be used to selectively energize one or more electric motors. Hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), fuel cell electric vehicles (FCEV), or purely electric vehicles (EV) can use the ESD as a source of at least a portion of the electrical energy needed for propelling the vehicle.
For the various vehicles listed above, the range of the vehicle is generally proportional to the amount of electrical energy that can be stored within the ESD. However, ESDs such as battery packs can contribute significant weight and mass to the vehicle, and can therefore affect the available range and overall size of the vehicle. Therefore, in certain vehicle designs a dedicated drive motor is connected to one or more drive shafts or axles. For example, an electric rear drive motor or eRDM system can selectively impart torque at various times to a rear drive axle in a front-wheel drive vehicle, with a conventional engine providing torque to the main or front drive axle independently of the operation of the eRDM system. A differential can be used to distribute the motor torque between a pair of independently rotatable rear drive axles, with the differential being suitably positioned to allow for forward packaging space for the potentially large ESD. Vehicle systems configured for electrically powering the vehicle in this manner can be referred to as electric drive motor systems, and in particular as electric rear drive motor (eRDM) systems when selectively powering the vehicle from the rear drive axles in a primarily front-wheel drive vehicle. While eRDM systems provide relative operating and packaging advantages over standard electric drive systems, the performance of such systems may remain less than optimal under certain operating conditions.